1. Field of the Invention
The present invention relates generally to distributed feedback semiconductor lasers and, more particularly, to a distributed feedback semiconductor laser with a .lambda./4 (quarter wavelength) phase-shifting structure.
2. Description of the Prior Art
A distributed feedback (hereinafter simply referred to as DFB) laser is one of the light sources used in long-haul and high-bit-rate transmission systems such as fiber optic communication, because it is easy to provide single longitudinal mode oscillation.
FIG. 1 is an enlarged perspective view schematically illustrating a known distributed feedback semiconductor laser. As shown in FIG. 1, this DFB laser comprises one conductivity type, for example, p-type GaAs semiconductor substrate 1 on which are sequentially grown an AlGaAs cladding layer 2, a GaAs active layer 3 and another conductivity type, for example, n-type AlGaAs guide layer 4. A diffraction grating 5 is formed on the guide layer 4. On the guide layer 4, there are sequentially grown the same conductivity type, for example, n-type AlGaAs cladding layer 6 and an n-type GaAs capping layer 7. The capping layer 7 undergoes the ion implantation using an ion such as a proton and a boron to form current restriction regions 8 of high resistivity at both sides of the stripe-shaped central portion of the capping layer 7. A pair of opposing electrodes 9 and 10 are respectively formed on the upper surface of the capping layer 7 and the rear surface of the semiconductor substrate 1 in ohmic contact therewith.
In the conventional DFB semiconductor laser having the uniform diffraction grating, there are two longitudinal modes with equal threshold gain in principle on both sides of the Bragg wavelength. There is then a possibility that this DFB semiconductor laser will operate in double lasing modes. In practice, it is frequent that the known DFB semiconductor laser operates in double lasing modes, causing so-called mode-hopping noise. To overcome this defect and to effect the oscillation in a single longitudinal mode, a DFB semiconductor laser with a .lambda./4 phase-shifting structure has been proposed. Also, various methods for realizing this improved DFB semiconductor laser have been proposed experimentally.
For example, as shown in FIG. 2, the phase-shifted DFB laser is realized by directly phase-shifting the diffraction grating 5 at its central portion of the waveguide by a distance of .lambda./4.
FIGS. 3A and 3B illustrate examples of further conventional phase-shifted DFB semiconductor lasers. According to the lasers shown in FIGS. 3A and 3B, the diffraction grating 5 is formed uniformly and the width of the stripe-shaped optical waveguide is changed at its central portion (w1.noteq.w2).
FIG. 4 further illustrates another example of the conventional phase-shifted DFB semiconductor laser. In this DFB semiconductor laser, as shown in FIG. 4, the central diffraction grating portion of the diffraction grating 5 is removed to form a smooth or flat region 11 and this flat region 11 is used to carry out the effective phase shift.
The above methods for effecting the .lambda./4 phase shift have the following problems. In the method described in connection with FIG. 2, the process for forming the diffraction grating 5 becomes complicated and it is difficult to form the diffraction grating 5. In addition, there is a problem that there are respectively formed an area in which the diffraction grating is not formed., and an area in which the diffraction grating is disturbed. In this case, if the .lambda./4 phase-shifting diffraction grating is formed, there occurs no problem.
According to the methods shown in FIGS. 3A and 3B, with respect to the transverse modes a and b in the x and y (horizontal and vertical direction as shown in FIG. 1), the mode distribution in the x direction is changed as shown in FIG. 5B (a solid line represents the mode distribution of a diffraction grating region 13 having the width w1 and a dashed line represents the mode distribution of the phaseshifting region 12 having the width w2). Then, the resultant effective refractive index difference .DELTA.N between the diffraction grating region 13 and the phase-shifting region 12 is used to carry out the effective phase shift. Essentially, these methods shown in FIGS. 3A and 3B are techniques that are only applicable to the refractive index guide type DFB semiconductor laser. The mode distribution in the y direction is not changed substantially as shown in FIG. 5A.
According to the method shown in FIG. 4, the mode distribution in the y direction is changed as shown in FIG. 6A (a solid line represents the mode distribution of the diffraction grating region 13 and a dashed line represents the mode distribution of the phase-shifting region 12) and the resultant effective refractive index difference .DELTA.N is used to carry out the effective phase-shift. However, because of the flat phase-shifting region 12 in which the diffraction grating is not formed, the coupling is weakened. If this method is applied to a refractive index guiding type semiconductor laser structure (buried laser, channeled substrate planar laser, ridge-waveguide laser and rib-waveguide-stripe laser, etc.), the mode distribution in the x direction is therefore changed as shown in FIG. 6B. Thus, the transverse mode is considerably changed at the boundary portion of the phase-shifting region 12 so that reflection loss occurs, etc. Particularly when this method is applied to the rib-waveguide-stripe DFB laser shown in FIG. 7, it becomes difficult to provide a difference in the thickness between the phase-shifting region 12 and flat portions 14 at the both sides of the phase-shifting region 12. As a result, the waveguide mechanism is changed in the phase-shifting region 12 from the refractive index guiding type to the gain guiding type, thus producing optical scattering loss.
Further, according to this method, if the pitch of the diffraction grating 5 is determined, the difference h in the effective thickness of the guide layer between the diffraction grating region 13 and the phase-shifting region 12 is inevitably determined so that the effective refractive index difference N is also determined. Thus, since .DELTA.N.times.l =.lambda./4, the length l of the phase-shifting region 12 cannot be selected freely.